chrono speed, vs temperature

[D Scott]

Hi Im new to reloading, and would appreciate some feedback about some of my chrono'd speeds Im getting with my reloads.

I''m shooting a Savage LRP 260- remmington, I have been shooting Nosler 140 custom comp HPBT, with 45 g of H4831 sc, cci br2 primers, remm brass, doing load development shooting at 100 yards I have been keeping groups at or under a half inch, at 300 yards groups around 2.5 inches. my extreme spread has been about 14-15 fps, with the past 60 rounds., average MV at 2776 fps

Today I shot 3 groups of 15 (45) rounds with new remm brass with the same load as above with very different velocity results, the temp was about 10 degrees cooler, and the pressure approx the same from the last session

fist group 2865 mv, with ES of 41
2nd group 2801 mv ES 57
3rd group 2766 mv ES39
I have been meticulous with brass prep and weighting each load precisely on a beam scale assuming its not a faulty chrono (chrony master-beta) great name, it was very windy today though

can muzzle velocity shift as the ambient temperature of the cartridge shifts to a higher or lower temp? would that change the ES ? would you expect a shift of over 100 fps from a change of 20 degrees or less?

any input would be greatly appreciated .

thanks in advance Scott

 

[MagnumManiac]

Unfortunately, internal ballistics is not an exact science, and yes changes in temperature can and do affect velocities and pressure.
Generally, the higher the temp, the higher the pressure and therefore velocity, but it has been known that very cool temps can also increase pressure.
Atmospheric conditions have little affect on internal ballistics, there is some, but almost non measurable. External ballistics is another story.
Barrel heat could also be a factor, so keep this in mind when developing a load.

What is more likely, is that the light on the day was different from the previous session, and this is what changed the reading on your chrony. This is very common.
I have the same chrony as you, and I always use the diffusers, no matter if it's cloudy, sunny or bits of both.

gun)

 

[Kevin Thomas]

D Scott,

The general rule for IMR type extruded tubular single base powder is approximately 1.7 fps per degree, in the direction of the temerature. That is, temp goes up ten degrees, you can expect velocity to increase about 17 fps. Temp decreases the same amount, and you'll see the same in the other direction. For the 20 degree shift you mentioned, I wouldn't be a bit surprised to see a 34 fps switch (or thereabouts), but would be looking for another explanation for a change of 100 fps. This is a general rule, and there are some exceptions.

Given the focus on the temperature sensitivity, this is becoming less of an issue today, but the trend remains the same.

 

[D Scott]

Thanks to both of you for your reply, I am going to try a different chrony, Ive read that the CED M2 is good for the money, any input here? I have a feeling the chrony is just a little to random at times.,
thanks again Scott

 

[Kevin Thomas]

Scott,

Oehler sets the gold standard in the chronograph field. The Model 35P (if memory serves) is the latest. Ken's not as active as he has been in years past, and these can be tough to get these days. Usually, when he does a run of them, they are sold before the first one gets assembled. Worth taking a look at, though, especially if you want something with a proof channel.